A Pulse Amplitude Modulation (PAM) receiver detects symbols in a received signal that are encoded as pulses having varying amplitude. In a PAM-N receiver, the received signal is compared to N−1 decision boundaries (e.g., reference levels) to detect one of N possible symbols. To accurately detect the symbols, each of the N−1 reference levels should be adjusted and aligned with a desired boundary. The optimal boundaries for symbol decisions are the N−1 vertical eye centers. However, the centers of the N−1 eyes of received PAM-N signal may be non-uniformly distributed and depend on the data pattern, inter-symbol interference, DC offset, equalizer adaption, analog front end (AFE) gains, and AFE nonlinearity. Furthermore, the centers of the N−1 eyes may vary with supply voltage and temperature. These factors make it difficult to maintain reference voltages at or near their optimal values. Poor alignment of the reference voltages may lead to frequency or phase lock failure.